Many transmission schemes encode data by modulating the amplitude and phase of a carrier so as to represent a symbol within a constellation diagram of the type shown in FIG. 1. The constellation diagram has axes extending in the real and the imaginary directions. The real and imaginary directions are orthogonal. In practical terms, if the real direction is represented by a sinusoid sin(ωt) then the imaginary is represented by
      sin    ⁡          (                        ω          ⁢                                          ⁢          t                +                  π          2                    )        =            cos      ⁡              (                  ω          ⁢                                          ⁢          t                )              .  Therefore an arbitrary symbol, such as that designated 2 can be represented by a suitable combination of the signals sin ωt and cos ωt. Modulators suitable for doing this are known as I-Q modulators and an example of such a modulator is shown in FIG. 2. It works well but exhibits significant output noise which tends to become exacerbated when trying to run at increased output power because of the greater currents required to drive the gates of the switching transistors.